My clutch has recently started playing up. Seems fine when you first start the bike, but after a few miles the clutch starts coming further and further back to the bar before it will disengage, eventually not disengaging at all, resulting in drag and difficultly changing gear.

There is no slip I can discern. I am going away on the bike shortly, so I need a quick solution.

What do I need to check? Is this simply a case of bleeding the clutch, or is this likely to be something more severe, perhaps a leak somewhere.

Here you go Herb, it's a long thread, but it's all there (see below). If in doubt, it's a quick check you can do that won't cost you anything. I use a 0.5mm jet. The 0.3mm blocks too easy and when it does it makes the bike feel like I'm doing clutchless shifts. The blocking is temperature and oil viscosity dependent and the jet size is a trade off between crunchy shifting or semi-engaging (dragging) clutch when stationary (worst when engine is cold). 0.5mm with 10W50 oil in 0-30C weather optimal IMO.

Me as well recommend you to have a read through the thread that 'limeyduffa' put up the link to.
Read it and yu will learn a lot about the clutch oil jet, Oberon slave cylinder, type of oil to use in the system and more.

Nice fix. Funny how Ktm and Ducati can't seem to sort out such a dead simple part like a slave cylinder. Keep an eye on the clutch master cylinder as well, mine got weepy around 30k miles and after reading about issues around the forums I ended up just replacing with another model from magura. No issues since.

its because almost every part on a modern vehicle is built to 'just' do the job at the lowest price. The Oberon is a quality part, that probably costs Oberon 20 times as much to procure as the part KTM fit as standard.

Reminds me of the John Glenn quote:

‘I felt exactly how you would feel if you were getting ready to launch and knew you were sitting on top of 2 million parts — all built by the lowest bidder on a government contract.’

Great with the Oberon slave cylinder!
Me as well replaced the original slave to an Oberon, 'some miles' ago - and at the same time I changed the mineral oil to Castrol Front Fork oil 5W.
The engine and the original master slave has been running for more than 57000 km.

I have another, seemingly unrelated issue with the wiring to the ignition switch now. I could scream with frustration right now. I'll start another thread, but basically the bike was dead when I move it from the garage. A bit of investigation and it seems that the wire to the side of the ignition barrel must be damaged. If I put the slightest pressure on this wire, everything comes on. Take the pressure off, and electrics go off again.